Clay Minerals (1967) 7, 155.
CATION-DIPOLE
CLAY
ORGANIC
INTERACTIONS
IN
COMPLEXES
K. K. B I S S A D A , W. D. J O H N S AND F. S. C H E N G
Department o[ Earth Sciences, Washington University,
St Louis
(Received 8 May 1967; revised 30 June 1967)
ABSTRACT:
Quantitative gas chromatographic analyses supplemented by
X-ray diffraction studies of the adsorption of ethanol and acetone (as model polar
organic compounds) on homoionic montmorillonite revealed marked variation
in the n u m b e r of molecules associated with each exchange cation. The results
show increasing association in the order K + ~ N a + ~
Ba 2+ ~ C a z+. K + a n d
N a + associate with two and three molecules, respectively, of e i t h e r ethanol or
acetone, and the resulting complexes expand to form a monolayer (,-.~13 A). Ba 2+
and Ca z+ form both monolayer complexes as well as double layer complexes.
In the single layer complexes Ba 2+ associates with either four molecules of
ethanol or four molecules of acetone, CaZ+ associates with five molecules of
ethanol or four molecules of acetone. In the double-layer complexes the observed
cation-molecule ratios are 1 : 8 for both Ba2+--ethanol a n d Ba2+-acetone, I : 10
for Ca2+--ethanol, and 1 : 8 for CaZ+-acetone.
The striking dependence of ethanol and acetone adsorption on the nature
of the exchangeable cation suggests that cation-dipole interactions play a n
important role in the adsorption process. Structural models of the organic
complexes are presented.
INTRODUCTION
Interest in the study of organo-silicate chemistry has expanded very rapidly in recent
years. The extent of this interest cart best be seen by reference to the recent reviews
by MacEwan (1962).
Studies of montmorillonite organic complexes have indicated that small polar
molecules are adsorbed between silicate sheets in mono- or multimolecular layers,
and that larger molecules may assume orientations with high angles of inclination
to the silicate sheet surfaces.
Since imbibition is generally confined to polar molecules, it has been more or
less assumed that the adsorption mechanism proceeds through dipole-surface interactions, supplemented in the case of compounds coetaining hydroxyl-, carboxyl- or
amino-groups, by O---H . . . . . O or N - - H . . . . . O bonding to the surface (Emerson,
1957; Brindley & Ray, 1964; Brindley & Moll, 1965; Brindley, 1965). As a result,
there has been a tendency to discount the effects of interlayer cations on the
156
K. K. Bissada et al.
adsorption process, except, perhaps, insofar as they activate the basal oxygen surface
(Barshad, 1952), by inducing ' . . . large dipole moments on the interlayer surfaces'.
It has been observed often that water imbibition in montmorillonite is strongly
influenced by exchangeable cations (Hendricks, Nelson & Alexander, 1940; Meting,
1946; Mooney, 1951; Mooney, Keenan & Wood, 1952; van Olphen, 1965). This is
manifested by variations in interlamellar expansion and measurements of 'hydration
number'. In these cases there has been no hesitancy in talking about the hydration
of cations.
Recently, Benson & King (1965) have shown that adsorption of organic vapours
on zeolites is caused by interactions of the molecules with the local electrostatic
fields of the exchangeable cations in the zeolite structures. Ahrens' (1966) correlations of ionization potentials, ionic charge and ionic size with stability indices of
amino acid-metal complexes, formed in aqueous solutions, indicate the importance
of ion--dipole interactions in such systems.
It appears likely that in montmorillonite, as in the zeolites, cation-dipole interactions are to be expected because of electrostatic attraction between cations and
the organic dipoles. We should expect, as a result, variations in the number of
molecules associated with each exchange site when occupied by different cations.
Conversely, demonstrated interdependence of quantitative adsorption and exchangeable cations would be indicative of such ion--dipole interactions in montmorillonite.
That such studies have not been carried out is due for the most part to analytical
difficulties in assessing quantitatively the number of molecules of any particular
organic species adsorbed under some sort of equilibrium conditions.
Quantitative data obtained in this laboratory have confirmed the importance
of such ion--dipole interactions involving ethanol and acetone with exchangeable
K +, Na +, Ba 2+, Ca ~+ in homoionic montmorillonites. Ethanol, whose source of
polarity lies in the hydroxyl group, was selected first as a model polar compound.
Adsorption isotherms, based on gas chromatographic analyses of dilute solutions
of ethanol in dodecane following equilibration with homoionic montmoriUonite,
were used to obtain the number of organic molecules associated with each cation.
The results show increasing association in the order K + < Na + < Ba ~+ < Ca 2+
for these cations whose ionic radii are 1-33, 0"97, 1"35 and 1-01 A, respectively.
EXPERIMENTAL
PROCEDURES
AND RESULTS
Characterization o] montmorillonite
The montmorillonite used for this study was from Belle Fourche, South Dakota
(supplied by the American Colloid Company). It has the calculated approximate
structural formula (Kerr et al., 1950):
(Nat.24, ? 0 " 0 4 ) (All.63 Feo.17 Mgo.25) (Alo.o7 Si3.93)Olo (OH)2The less than one micron (equivalent spherical diameter) fraction was separated
by sedimentation procedures and utilized entirely in this study. No impurities could
be detected by X-ray analysis.
Clay organic complexes
157
Homoionic montmorillonite with Na +, K +, Ca 2+ or Ba 2+ as exchangeable ions,
was prepared from the fractionated stock suspension by stirring portions of the
suspension with batches of Duolite C-20 cation exchange resin in the appropriate
cationic form. The amount of resin used was equivalent to ten times the total
exchange capacity of the clay suspension. This batch operation was repeated four
times, followed by sieving to remove the resin. As an additional precaution against
salt contamination the homoionic suspension was subsequently dialysed. The sluries
were allowed to dry at 40 ~ C in wide, fiat-bottomed polyethylene trays, forming
clay films. The films were cut into small flakes (3 x 3 mm), which were used in
the subsequent adsorption studies.
Cation exchange capacity
Total cation exchange capacity was determined by sodium analyses of a sample
of homoionic Na-montmorillonite (less than 1 ~ fraction), utilizing non-destructive
neutron activation analysis. The measured total cation exchange capacity was
100 meq/100 g (+_3"0 meq). This total exchange capacity includes contributions
from exchange sites on the intedamellar surfaces, as weU as sites on the edges of
the particles. Since the former contribution amounts to approximately 15-20
meq/100 g (Williams, 1963), we would expect interlamellar exchange to amount
to about 80-85 % of the total.
Previous experience has indicated that bulky organic cations associate only with
interlammellar sites. Accordingly, a Na+-saturated sample of fractionated montmorillonite was treated four times with a large excess of pyridine hydrochloride
solution. The pyridinium-montmorillonite so obtained was easily washed free of
excess salt, air dried and lightly ground with a spatula. Care was taken to avoid contamination by any organic matter. Samples, 0-8 mg, were analysed in duplicate for
their carbon content and, therefore, their pyridinium content, using an F & M*
model 185 C, H, N analyser. The intertamellar exchange capacity measured was
85 meq/100 g ( +_2 meq). This amounts to an interlayer charge of 0"33 per --Olo (OH)~
structural unit, essentially identical to the ideal charge suggested by Ross & Hendricks
(1945), or za units of charge per unit cell.
Organic adsorption experiments
Pure anhydrous ethanol was obtained by refluxing the 95 % alcohol over calcined
CaO, with subsequent distillation.
Pure acetone was obtained by re-distilling the 'spectranalysed" grade (Mathieson
Chemical Co.) over dried K2CO~.
Olefin free, 99 + %, n-dodecane (Mathieson Chemical Co.) was used as received
as an 'inert' solvent for ethanol and acetone.
Approximately 1"1 g of homoionic montmorillonite flakes were introduced into
10 ml tared glass ampules (Kimble 'color-breaker') dehydrated at 225 ~ C over* F & M Scientific Corp.
158
~K. K. Bissada et al.
night, and re-weighed after cooling over P205, to obtain the weight of dried clay.
Solutions with the desired concentrations of ethanol or acetone in n-dodecane were
prepared in 50-ml bottles, fitted with puncture-top plastic screw caps and rubber
diaphragms lined with aluminium foil. Five-miUilitre aliquots of each stock solution
were transferred by means of a hypodermic syringe to each of the series of ampules
containing the homoionic montmorillonite, and the weight of added solutions determined. The ampules were then heat-sealed while dipped in dry ice. Similarly, two
5-ml aliquots of solution were transferred to empty tared ampules which were
heat-sealed. The latter served as blanks and, when subsequently analysed, represented the initial concentration of the solutions. The ampules were then placed
in a water bath at 25 ~ + 0.5 ~ C for 48 hr to equilibrate with the clay.
The equilibrium concentrations of ethanol or acetone were determined by gas
chromatography, using n-propanol and methyl ethyl ketone, respectively, as internal
standards. An F & M model 700 gas chromatograph, with a thermal conductivity
detector and a carbowax 20-M column was used. The differences between the
initial concentrations (determined from the mean of two blanks) and the finsl
equilibrium concentrations for each solution were taken as the quantity of ethanol
or acetone adsorbed by the montmorillonite, assuming that n-dodecane is not being
adsorbed concurrently. It should be emphasized here that the quantities of ethanol
or acetone indicated to be adsorbed by the clay are determined indirectly by
measuring their depletion from the respective solutions. This implies that concurrent
n-dodecane adsorption, if any, would have to be accompanied by an additional
finite quantity of ethanol or acetone over that assigned. Subsequent interpretation
of the results will show that the interlayer space available is just sufficient to
accommodate only the ethanol or acetone indicated; that is to say, alcohol or
acetone in excess of the amounts we are assigning, and in addition, dodecane, could
not be accommodated in the space available. Supplementary X-ray diffraction studies
revealed that n-dodecane alone exhibits no interlamellar imbibition.
The adsorption experiments were made on solutions ranging in initial concentration from about 5 to 21%. Adsorption isotherms were obtained by plotting
weight of ethanol or acetone adsorbed per gram of homoionic montmorillonite, as a
function of the final equilibrium concentration. The points on each set of isotherms
were determined over a period of about two months and the scatter is indicative of the
reproducibility at any one concentration (Fig. 1).
X-ray diffraction studies
In order to ascertain whether the adsorbed molecules are disposed in monolayer
or multilayer configurations in the interlayer region, the process was monitored
by X-ray diffraction methods.
A dehydrated, tiny oriented ribbon of each homoionic montmorillonite
(2 • 10 ram) was introduced, together with a very small amount of a 20% solution
of the organic compound in n-dodecane, into thin-walled (0"01 ram) glass capillaries.
The capillaries were sealed and their contents allowed to equilibrate at 25 ~ C.
Clay organic complexes
025,.-
159
0"25
r
~A 020,~"
__
ca'"
Ba"~
9
ca~
.~ 0.20
B a~
.
045
~ o.~5
Na+
9
<
o s o L ~
0
5
10
15
K "~
~o-so
5
10
20
25
0
Equil.ibrium concentration of ethanol. (wt %)
20
25
FIG. 1. Absorption isotherms for acetone--dodecane and ethanol-dodecane
montmorillonitesystemsat 25~ C.
Fibre X-ray diffraction patterns were obtained using a Debye-Scherrer camera and
filtered Fe K~ radiation. The (001) spacing for each complex was measured. K +- and
Na+-montmoriUonites at equilibrium with the organic solutions gave spacings
indicative of interlayer sorption of monomotecular layers. Ba z+- and CaZ+-mont morillonites formed two-molecular layer complexes.
In addition, oriented flakes of the homoionic montmoriUonites, which had
equilibrated with approximately 15% solutions of ethanol or acetone in dodecane,
were placed on glass slides and immediately the basal reflections were scanned
by an X-ray diffractometer (range 40-8 ~ 20, Cu K~). The flakes were sprayed with
the equilibrium solutions during the scanning runs. The K +- and Na+-mont moriUonite complexes gave (001) spacings of about 13 A for both ethanol and
acetone, indicative of monomolecular configurations. No change in the dool spacings
was observed after spraying was discontinued. In contrast, the Ba z+- and CaZ+-complexes gave dool spacings of about 17 A, characteristic of two-layer complexes.
When spraying was discontinued, a series of repeated runs showed gradual loss
of the 17 A reflection and gradual development of 13"5 A peak. The X-ray data
are summarized in Table 1.
Estimation of ethanol and acetone retained in monomolecular layers in
Ba- and Ca-montmoriUonite
The quantity of ethanol or acetone retained in the collapsed monolayer phases
of the Ca- and Ba-montmorillonite was determined as follows. Montmorillonite
flakes for which X-ray analyses had indicated collapse to a monolayer, were blotted
carefully with filter paper, placed in ampules, and weighed. About 5 ml water was
added to the contents of each ampule and the weight of water added was determined.
The ampules were sealed and placed in a bath at 70 ~ C for a few hours to 'extract'
adsorbed ethanol or acetone. The contents of the ampules were then centrifuged,
160
K. K. Bissada et al.
TABLE 1. X-ray data
Exchangeable
cation
Ethanol
Spacing (A)
Acetone
No. of layers
Spacing (A)
No. of layers
(a) (001) spacing for homoionic montmoriUonite in equilibrium with 15~o ethanol or acetonedodecane solutions in sealed capillaries
K§
13.0
1
13.4
1
Na +
13-5
1
13.2
1
Ba ++
17.2
2
17-3
2
Ca 2+
17'3
2
17.3
2
(b) (001) spacing for flakes of montmorillonite-organic complexes under ambient conditions
K+
Na +
Ba 2+
Ca ~+
13.1
13"4
16"7----->13-6
17.2------+13.7
1
1
2-->1
2--->1
13.2
13"3
17"0 ~ 13"3
17"2
~ 13"5
1
1
2--+1
2--->1
a n d the s u p e m a t a n t solutions a n a l y s e d b y gas c h r o m a t o g r a p h y for their e t h a n o l o r
a c e t o n e content. T h e a m o u n t s of w a t e r - e x t r a c t e d organics, c o r r e s p o n d i n g to e t h a n o l
o r a c e t o n e r e t a i n e d b y the m o n o l a y e r p h a s e s of the Ba- a n d C a - c o m p l e x e s a r e
given in T a b l e 2. B y a n a l y s i n g directly the a m o u n t of organics retained, this figure
includes b o t h t h a t a d s o r b e d on i n t e r l a y e r sites a n d on crystallite edges.
TABLE2. Ethanol and acetone retention in monomolecular layer in Ca ~+- and
Ba2+-montmorillonite
Ethanol
Total organic extracted (g/g clay)
Interlamellar retention (g/g clay)
Molecules/unit cell
Molecules/cation
Acetone
Ba2+
Ca2+
Ba~+
Ca~+
0.097
0.08
1-35
4.1
o. 129
0.11
1.77
5.3
0.114
0.10
1-34
4.0
o. 143
0.12
1.53
4-6
T a k i n g into a c c o u n t the a c c u r a c y of o u r c a t i o n exchange m e a s u r e m e n t s as
i n d i c a t e d earlier, we can establish that the edge c o n t r i b u t i o n to the t o t a l exchange
i o n site m u s t be within the range of 9 - 2 0 % . W e t a k e the m e a n value 15% as
b e i n g r e a s o n a b l e , in o r d e r to estimate the organics r e t a i n e d on the i n t e r l a y e r sites
only,
I t is i m p o r t a n t to e l a b o r a t e on the significance of the values for m o l e c u l e s / c a t i o n
given in T a b l e 2. T h e n a t u r e of the a l c o h o l a n d a c e t o n e e x t r a c t i o n e x p e r i m e n t s
was such t h a t s m a l l a m o u n t s of these c o m p o u n d s d i s s o l v e d in o c c l u d e d d o d e c a n e
Clay organic complexes
161
would also be included in the totals shown. Analysis of occluded dodecane indicates
that up to about 5 % of the extracted alcohol or acetone may be present external
to the clay crystaUites.
In short we would expect calculated molecule/cation figures to be slightly
higher than the true numbers. As a result of these uncertainties we are justified
only in concluding that the Ba- and Ca-ethanol and Ba- and Ca-acetone one-layer
complexes form with molecule/cation ratios of approximately 4, 5, 4 and 4, respectively. By way of comparison, Rios & Rodriques (1961) have likewise determined a value of 4 for the molecule/Ba z+ cation ratio, utilizing adsorption from
the vapour phase. The implications of these measurements are discussed in the next
section.
INTERPRETATION
AND DISCUSSION
The m a x i m u m amounts of ethanol or acetone adsorbed, as deduced from the
adsorption isotherms, are indicated in Table 3. These data, along with exchange
cation populations, l~ermit the computation of the number of molecules of ethanol
or acetone associated with each cation (Table 3). The data reveal the striking
dependence of the molecular association of ethanol or acetone upon the nature
of the exchangeable cation.
TAaLE 3. Acetone and ethanol adsorption on homoionic montmorillonite
No. of interlayer cations
Per unit cell
Per 3 cells
Formula weight of montmorillonite
Equilibrium quantity of organic compound adsorbed on montmorillonite
Ethanol
g/g clay
molec./unit cell
molec./3 cells
molec./cation
Acetone
g/g clay
molec./unit cell
molec./3 cells
molec./cation
K-mont
Na-mont
Ba-mont
Ca-mont
2/3
2
757
2/3
2
747
1/3
1
777
1/3
1
745
0"088
1-45
4.35
2"17
(2)
0-125
2.02
6,06
3"03
(3)
0-160
2-70
8.10
8-10
(8)
0-215
3.47
10.4
I0"4
(10)
0.096
1.25
3"75
1.88
(2)
0.174
2.23
6.69
3.34
(3)
0.198
2-65
7"95
7.95
(8~
0-203
2"60
7"81
7.81
(8)
Considering pairs of approximately equal size and different charge, the divalent
cations give rise to adsorption of larger numbers of alcohol or acetone molecules.
For ions of equal charge and differing radius, the smaller ions lead to greater
adsorption.
c
162
K. K. Bissada et al..
Differences in structure and in properties between ethanol and acetone seem to
have little effect on their adsorption behaviour. Ethanol with a dipole moment of
1.7 • 10 -18 esu. cm, contains, by virtue of its O H group, an electron donor atom
and an active hydrogen. It might be expected, therefore, to form O - - H . . . . . O
bonds with the oxygens of the silicate basal surface. Acetone with a dipole moment
of 2"86 • 10 -18 esu. cm, contains a donor atom but no active hydrogen atoms
associated with it, thus indicating that some mechanism other than O - - H . . . . . O
bonding is responsible for the adsorption of these polar organic molecules. Barshad
(1952) has shown that diethyl ether, with a dipole m o m e n t of 1"17 • 10 -18 esu. cm,
is Iikewise adsorbed by expanding lattice layer silicates, although it has no
O - - H . . . . . O bonding capability.
These observations indicate that cation-dipole interactions of an electrostatic
nature play an important role in the sorption process. We are led to conclude that
the polar molecules solvate the exchangeable cations and that the 'solvation
number' is related to the electrostatic field strength associated with each exchangeable cation, being related to both the charge and size of the latter. As shown by
Benson & King (1965), the electrostatic attractive energy between an ion and a
polar molecule can be defined by the expression:
~p= _
Ca + "/~r
r~
where Ca + is the cationic charge, t~p is the dipole moment of the polar molecule,
and ro is the interaction distance.
TABLE4. Theoretical cation-dipole interaction energies (~p)
Cation
K+
Na +
Ba~+
Ca z+
Ethanol
(t~ = 1.73 • 10-18 esu. cm)
to(A)
~bp(kcal/mole)
3-24
2.91
3-26
2-94
11-4
14"1
22"5
27"7
Acetone
(/z = 2.86 x 10-18 esu. cm)
to(A)
ffp (kcal/mole)
4.07
3.71
4-09
3-75
11.9
14.4
23"6
28-1
4,p values, computed for K +-, Na +-, Ba ~+- and Ca~+-ethanol and acetone interactions are given in Table 4. These are based on values of 1"73 • 10 - i s esu. cm and
2"86 • 10 -18 esu. cm for the dipole moments of ethanal and acetone, respectively
(dipole moments from McClellan, 1963). The interaction distance is taken as the
distance from the centre of the cation to the centre of the dipole (ionic radii from
Ahrens, 1964). The centres of the dipoles were located by calculating the centre
of the positive charge for the molecule, then determining the centre of negative
charge using the basic relation for the dipole moment (t~ = q.d). The interaction
Clay organic complexes
163
distance, therefore, could be estimated readily by assuming that the cations make
contact with the oxygen atoms of the organic molecules; in the case of acetone
\
the centre of the cation is assumed to be coaxial with the
/
C ~ O ; in ethanol, the
point of contact is assumed to be at the point of emergence of the lone pair resultant
of the tetrahedrally hybridized oxygen atom of the molecule. It is interesting to
note that the interaction energies for acetone and ethanol are about the same. This
is because the effect of the larger dipole moment of acetone is compensated for by
the shorter interaction distances in the cases of the ethanol complexes.
The 4~ values clearly predict an increase in complex stability in the order
K < Na < Ba < Ca, which corresponds to our order of observed solvation numbers.
K + and Na + associate with two and three molecules, respectively, of both ethanol
and acetone, and the homoionic clays expand to form single layer complexes. Ba 2+
and Ca ~+, on the other hand, are solvated at saturation by eight and ten ethanol
molecules, or eight and eight acetone molecules, respectively. We can speculate
that the solvation energy released in these latter cases is sufficient to promote and
permit more extensive interlayer expansion (to ,--,17 A) in opposition to the electrostatic attraction holding layers together via cation-silicate layer interaction. At less
than saturation the two-layer Ba- and Ca-complexes become unstable and adjust to new
equilibrium states containing only half the number of ethanol or acetone molecules,
thereby contracting to monolayer periodicities of 13-14 A.
Structural considerations
From the data obtained it is possible to denote the extent of silicate surface
coverage for each of the homoionic complexes. If cation--dipole interactions are
important as we suggest, we prefer to think in terms of the distribution of cationorganic complex groups, rather than unrelated distributions of cations and organic
molecules. Thus we have constructed structural models (Figs 2 and 3) in which
a cation-organic complex grouping is indicated, resulting from interaction of the
cation wfth the neutral organic dipoles. For convenience a structural segment is
pictured which includes an area equivalent to 9 unit cells. These models are
drawn to scale as indicated in Fig. 2 for the ethanol and in Fig. 3 for the acetone
complexes. These complex groupings were distributed over the available surface
so as to maintain the appropriate number of cations per unit cell. It becomes
apparent that the approximate integral numbers of molecules per cation (solvation
numbers) assigned are consistent with reasonable schemes of close packing. Particularly in the case of the Na- and K-complexes it is necessary to postulate specific
ethanol and acetone orientations in order to prevent significant steric overlap. The
ethanol molecule is oriented with its plane of symmetry perpendicular to the oxygen
substrate surface analogous to the orientations suggested by Brindley & Hoffmann
(1962) and Bradley, Weiss & Rowland (1963) for ethylene glycol in aUevardite
and Na-vermiculite respectively. The acetone molecules were assigned an orientation
164
K. K. Bissada et al.
!"
1
G0~--
FIG. 2. Structural models of K+-, Na+-, Ba2+- and Ca~+-montmorilloniteethanol
complexes.
that allows the double bond to lie parallel to the silicate sheet and the plane of
symmetry containing both methyl groups to be perpendicular to the substrate.
Another structural detail depicted in Figs 2 and 3, more speculative in character,
involves placement of each organic molecule in association with a hexagonal 'hole'
of the silicate substrate. This would permit further packing relative to the silicate
substrate and would permit the active methylene (ethanol) and methyl (acetone)
groups to participate in weak C--H . . . . . O bonding to the silicate oxygens, as
suggested by Bradley (1945), MacEwan (1948) and Hoffmann & Brindley (1960).
Clay organic complexes
165
I-,
I
--6.1~--
i"
FIG. 3.
Structural models of K§
-4.1A-
"7'.
N a +-, Ba z+- and Ca=+-montmorillonite acetone
complexes.
This would also explain the discrepancy between van der Waals thicknesses for
ethanol (5"0 A) and acetone (6.1 A) and the respective A values, 4-2 A and 4"1 A.
Before we take these schematically depicted structures too seriously we need to
recall that they are based on the assumption of exact whole number solvation
numbers. They imply also a regularity of cation distribution and conformity to
substrate symmetries which may indeed not exist, if the interlayer organization is
as quasi-liquid as is often presumed.
166
K. K. Bissada et al.
It does seem clear f r o m these models, however, t h a t the a v a i l a b i l i t y of a p p r o p r i a t e
surface a r e a is the p r i m e f a c t o r in limiting the s o l v a t i o n n u m b e r s of these complexes.
I n the t w o - l a y e r c o m p l e x e s of Ba- a n d C a - m o n t m o r i l l o n i t e with acetone or
e t h a n o l , it is a p p a r e n t that in every instance t h e cations h a v e s o l v a t i o n n u m b e r s
d o u b l e those of the single-layer complexes. W e p r e s u m e that the t w o - l a y e r c o m p l e x
groups are f o r m e d b y the SUl~erposition of a n identical second l a y e r of e t h a n o l o r
a c e t o n e on the respective single layers which we described, the cations still m a i n taining a central p o s i t i o n b e t w e e n their solvating molecules.
ACKNOWLEDGMENTS
We wish to acknowledge support for this study by National Science Foundation Grant
GP-4880.
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